1. Field of the Invention
The present invention relates to an acceleration sensor for detecting an acceleration of a vehicle and the like, and more particularly to improvements of the acceleration sensor both in mechanical strength and sensitivity thereof.
2. Description of the Prior Art
An acceleration sensor is used for the purpose of detecting an acceleration caused by an impact (or centrifugal force) of colliding vehicles. The detected acceleration is used to control the operation of a suitable system, for example such as an air bag system and the like. In a typical one of conventional acceleration sensors of this type, a piezo-resistance device is mounted on a cantilever member of the sensor. When the sensor is subjected to acceleration, the cantilever member deflects due to such acceleration, thereby causing a change in the resistance value of the piezo-resistance device. The change enables the sensor to detect the acceleration. In other words, for example, disclosed in Japanese Patent Laid-Open No. Hei 5-281251 is an example of a four-sided beam type acceleration sensor, in which example: a square frame formed from a silicon plate has a beam member extending from each side toward a center of the frame and connected with a center weight portion thereof; and, a piezo-resistance device is formed in a surface of each of the beam members through impurity-diffusion processes.
When the acceleration sensor as described above is subjected to acceleration, resistance values of the piezo-resistance devices vary in response to strains caused in the devices by the deflections of the beam members which enable the sensor to detect the acceleration.
However, in the above-mentioned conventional acceleration sensor, since the strains of the piezo-resistance devices are caused by the deflections of the beam members, sensitivity of the sensor depends substantially exclusively on flexibility of each of the beam members with respect to acceleration. In other words, in order to improve the sensitivity of the acceleration sensor, it is necessary to reduce each of the beam members in thickness, thereby improving the beam members in flexibility. The beam members thus improved in flexibility may sufficiently deflect even when the sensor is subjected to a minute acceleration, which enables the piezo-resistance devices of the beam members to have resistance values change without fail. In this case, since the beam members are reduced in thickness, the sensor is poor in mechanical strength. This is a problem inherent in the conventional acceleration sensor.